The Road Safety Authority (RSA) said an observational study also revealed motorists in Cork city were twice as likely to be seen with a mobile in their hand, with one in ten offenders spotted.

RSA's Noel Brett launched a new safety campaign and appealed for road users to be aware of the risks associated with using a phone while driving.

"The results of this study are extremely worrying, particularly when we know that you are four times more likely to crash if you use a hand held mobile phone while driving," said Mr Brett.

"We also know that the results understate the true extent of the problem as the study did not capture those texting while driving or those using a hands-free kit.

"In fact, research tells us that using a hands-free mobile phone is no safer than using a hand-held mobile phone while driving."

Mobile phone use while driving became a penalty point offence in September 2006. and is now the second highest penalty point offence in Ireland after speeding. A total of 93,525 offences have been recorded - including 16,000 in the year up to August 30.

The RSA is targeting drivers with a radio ad warning to them to 'Switch Off, Before You Drive Off'.

Garda Assistant Commissioner, John Twomey, said it is unsafe and illegal to use a hand-held mobile phone while driving.

"If caught, a driver must pay a fixed charge notice of €60 and will incur two penalty points. If a driver does not pay the fixed charge and is convicted in court, four penalty points and a fine of up to €2,000 will be applied," he warned.

The RSA observations revealed:

- Over 1 in 10 drivers (12pc) were observed using a phone in Cork, followed by 9pc in Maynooth.

- In two separate sites in Maynooth and Cork, almost one in five (16pc) drivers were spotted.

- The lowest levels of phone use while driving were in Drogheda and Dundalk at 3pc and Athlone, just 2pc.

- The rate of mobile phone use in Limerick was 7pc, and was 4pc in both in Galway and Dublin.

The RSA study was conducted in December 2009 at 45 locations in seven counties across Ireland. Of 33,949 drivers observed, 1,964 were using a hand-held mobile phone while driving, representing over 1 in 20 drivers (6pc).